Method and system for operating a quantum network node

Abstract

Method for operating a quantum network node comprising an electron spin host providing an electron spin for a communication qubit and a nuclear spin host providing a nuclear spin for a data qubit. The method comprises: data qubit state preparation; entanglement, comprising subjecting the electron spin to a protocol comprising multiple repetitions of a primitive for entangling with another quantum system; and data qubit state use, comprising performing operations on and/or readout of the data qubit state or causing entanglement between the data qubit state and a further quantum system. When the wanted (unwanted) state of the electron spin host is a wanted (an unwanted) charge state, the protocol comprises charge state resetting. The method may also or alternatively comprise determining an average unwanted state electron spin value when the electron spin host is in the unwanted state, and further steps in the entanglement protocol.

1 . A method for operating a quantum network node, wherein the node comprises
 a wide-bandgap solid state material comprising at least one electron spin host providing an electron spin for a communication qubit and at least one nuclear spin host providing a nuclear spin for a data qubit, the electron spin and the nuclear spin being magnetically coupled;   wherein the method comprises sequential steps of:
 data qubit state preparation, comprising preparing the nuclear spin in a predetermined nuclear-spin quantum state; 
 entanglement, comprising subjecting the electron spin to an entanglement protocol comprising multiple repetitions of an entanglement primitive (EP, EP 1 ) for entangling with another quantum system; and 
 data qubit state use, comprising performing operations on and/or readout of the data qubit state or causing entanglement between the data qubit state and a further quantum system; 
   wherein the electron spin host occupies a wanted state of the electron spin host (NV − ), or an unwanted state of the electron spin host (NV 0 );   wherein
 the wanted state is a wanted charge state and the unwanted state is an unwanted charge state, and the method comprises that the entanglement protocol comprises a step of charge state resetting; resetting a charge state (NV − ; NV 0 ) of the electron spin host from the unwanted charge state to the wanted charge state; and/or 
 the method comprises a step of determining an average unwanted state electron spin value of the electron spin when the electron spin host is in the unwanted state of the electron spin host (NV 0 ), and
 wherein the entanglement protocol comprises a step of varying electron spin states (ms=−1, 0, +1) in a series of the entanglement primitives (EP 1 ), providing an average series electron spin value of the series of entanglement primitives in accordance with the average unwanted state electron spin value. 
 
   
     
     
         2 . The method according to  claim 1 , wherein the method comprises that the entanglement protocol comprises the step of charge state resetting, and
 wherein the step of charge state resetting comprises irradiating the electron spin host with electromagnetic radiation promoting or causing the change of the charge state of the electron spin host from the unwanted charge state to the wanted charge state.   
     
     
         3 . The method according to  claim 2 , wherein the data qubit has a coherence time, and the method comprises performing the step of charge state resetting in a time interval shorter than the coherence time. 
     
     
         4 . The method according to  claim 1 , comprising selecting a portion of the solid-state material comprising the nuclear spin host and the electron spin host, wherein
 the nuclear spin host and the electron spin host have a predetermined coupling strength (J), wherein the coupling strength (J) is between 100 Hz and 100 MHz; and/or   in the portion of the solid-state material the concentration of nuclear spin hosts is between 0.01% and 1%.   
     
     
         5 . The method according to  claim 1 , wherein the entanglement protocol comprises in or between plural repetitions of entanglement primitives (EP) the step of charge state resetting or at least a step of attempting charge state resetting. 
     
     
         6 . The method according to  claim 1 , wherein the entanglement protocol comprises the step of performing a status check, comprising determining a charge state of the electron spin host, and wherein the entanglement protocol comprises selectively executing or not, on the basis of the status check, the step of charge state resetting. 
     
     
         7 . The method according to  claim 6 , wherein the status check comprises detecting a predetermined number of photons indicative of one or more electron spin states of the wanted charge state of the electron spin host. 
     
     
         8 . The method according to  claim 1 , wherein during the step of entanglement, the nuclear spin is repeatedly subjected to controlled rotation. 
     
     
         9 . The method according to  claim 1 , comprising the step of determining an electron spin state of the electron spin host when the electron spin host is in the unwanted state (NV 0 ). 
     
     
         10 . The method according to  claim 1 , wherein the method comprises the step of determining an average unwanted state electron spin value of the electron spin when the electron spin host is in the unwanted state of the electron spin host (NV 0 ), and wherein the entanglement protocol comprises the step of selecting different electron spin states (m s =−1, 0, +1) as mutually different basis states (|0>, |+1>; |−1>, |0>) for a series of the entanglement primitives (EP), providing an average series electron spin value of the series of entanglement primitives (EP) in accordance with the average unwanted state electron spin value. 
     
     
         11 . The method according to  claim 10 , wherein the step of entanglement comprises the step of driving electron spin transitions between different electron spin states of the electron spin when the electron spin host is in the unwanted state of the electron spin host (NV 0 ) for controlling the average unwanted charge state electron spin value. 
     
     
         12 . The method according to  claim 1 , wherein the solid-state material comprises or is one selected from the group consisting of diamond, silicon carbide, silicon, and 2-dimensional material, and
 wherein the electron spin host is formed by an optically active defect site in the solid-state material and the nuclear spin host is formed by an isotope atom in the solid state having a nonzero nuclear spin.   
     
     
         13 . A system for operating a quantum memory or quantum network node, comprising:
 a wide-bandgap solid state material comprising at least one electron spin host providing an electron spin for a communication qubit and at least one nuclear spin host providing a nuclear spin for a data qubit, the electron spin and the nuclear spin being magnetically coupled;   a first electromagnetic radiation system for manipulating the electronic spin host;   a second electromagnetic radiation system for manipulating the nuclear spin host;   a control system operably connected with the first electromagnetic radiation system and the second electromagnetic radiation system;   wherein the electron spin host occupies a wanted state of the electron spin host (NV − ) in which the electron spin is available for an entanglement primitive (EP), or an unwanted state of the electron spin host (NV 0 ) in which the electron spin is not available for the entanglement primitive (EP);   wherein the system further has means configured to execute a method comprising sequential steps of:
 data qubit state preparation, comprising preparing the nuclear spin in a predetermined nuclear-spin quantum state; 
 entanglement, comprising subjecting the electron spin to an entanglement protocol comprising multiple repetitions of the entanglement primitive (EP, EP 1 ) for entangling with another quantum system; and 
 data qubit state use, comprising performing operations on and/or readout of the data qubit state or causing entanglement between the data qubit state and a further quantum system; 
   wherein the electron spin host occupies a wanted state of the electron spin host (NV − ), or an unwanted state of the electron spin host (NV 0 );   wherein
 the wanted state is a wanted charge state and the unwanted state is an unwanted charge state, and the method comprises that the entanglement protocol comprises a step of charge state resetting: resetting a charge state (NV − ; NV 0 ) of the electron spin host from the unwanted charge state to the wanted charge state; and/or 
 the method comprises a step of determining an average unwanted state electron spin value of the electron spin when the electron spin host is in the unwanted state of the electron spin host (NV 0 ), and 
   wherein the entanglement protocol comprises a step of varying electron spin states (ms=−1, 0, +1) in a series of the entanglement primitives (EP 1 ), providing an average series electron spin value of the series of entanglement primitives in accordance with the average unwanted state electron spin value.   
     
     
         14 . The system according to  claim 13 , further comprising an electromagnetic radiation system for irradiating the electron spin host with resonant electromagnetic radiation for resetting a charge state of the electron spin host from the unwanted charge state (NV 0 ) to the wanted charge state (NV − ). 
     
     
         15 . The system according to  claim 13 , wherein the solid-state material comprises diamond, the electron spin host is an optically active defect site in the diamond, selected from a group consisting of a nitrogen—vacancy (NV) center, a silicon—vacancy (Si—V) center, a lead—vacancy (Pb—V) center or a tin—vacancy (SN—V) center; and
 wherein the nuclear spin host is formed by a  13 C atom. 
 
     
     
         16 . (canceled) 
     
     
         17 . A non-transitory computer readable storage medium having stored thereon which when executed by a processor operate a quantum network node, wherein the node comprises a wide-bandgap solid state material comprising at least one electron spin host providing an electron spin for a communication qubit and at least one nuclear spin host providing a nuclear spin for a data qubit, the electron spin and the nuclear spin being magnetically coupled;
 wherein the instructions comprise sequential steps of:   data qubit state preparation, comprising preparing the nuclear spin in a predetermined nuclear-spin quantum state;   entanglement, comprising subjecting the electron spin to an entanglement protocol comprising multiple repetitions of an entanglement primitive (EP, EP 1 ) for entangling with another quantum system; and   data qubit state use, comprising performing operations on and/or readout of the data qubit state or causing entanglement between the data qubit state and a further quantum system;   wherein the electron spin host occupies a wanted state of the electron spin host (NV − ), and an unwanted state of the electron spin host (NV 0 );   wherein
 the wanted state is a wanted charge state and the unwanted state is an unwanted charge state, and the method comprises that the entanglement protocol comprises the step of charge state resetting: resetting a charge state (NV − ; NV 0 ) of the electron spin host from the unwanted charge state to the wanted charge state; and/or 
 the method comprises the step of determining an average unwanted state electron spin value of the electron spin when the electron spin host is in the unwanted state of the electron spin host (NV 0 ), and 
   wherein the entanglement protocol comprises the step of varying electron spin states (ms=−1, 0, +1) in a series of the entanglement primitives (EP 1 ), providing an average series electron spin value of the series of entanglement primitives in accordance with the average unwanted state electron spin value.   
     
     
         18 . The method according to  claim 5 , comprising irradiating the electron spin host with electromagnetic radiation promoting or causing the change of the charge state of the electron spin host from the unwanted charge state (NV 0 ) to the wanted charge state (NV − ). 
     
     
         19 . The method according to  claim 8 , comprising repeatedly subjecting the nuclear spin to a decoupling sequence. 
     
     
         20 . The system according to  claim 13 , wherein the other quantum system is another quantum network node. 
     
     
         21 . The method according to  claim 2 , wherein the electromagnetic radiation comprises resonant optical radiation.